Field of the Invention
The invention relates to a piezoelectric element having a first and a second stack containing a plurality of piezoelectric ceramic layers and having an intermediate layer disposed between the first and second stacks. Electrodes are disposed in each stack between the respective ceramic layers and on the respective outer side of the ceramic layers bounding the stack.
A piezoelectric element of this type is disclosed in Published, Non-Prosecuted German Patent Application DE 35 18 055 A1 and in German Patent DE 34 34 726 C2. In order to operate the piezoelectric element, the electrodes have a voltage applied to them in such a way that the piezoelectric ceramic layers of the first stack respectively expand, while the piezoelectric ceramic layers of the second stack respectively contract, or vice versa. This leads to the bending of the piezoelectric element, which can be utilized to operate a mechanical or electrical switch. The intermediate layer disposed between the two stacks is likewise a ceramic layer, but this itself does not execute any active movement. For this purpose, the ceramic of the intermediate layer is either unpolarized and is consequently not piezoelectrically active, or else it is not subjected to any electric field when the piezoelectric element is driven. The intermediate layer serves to balance out the opposite movements of the two stacks and therefore increases the service life of the piezoelectric element.
A piezoelectric element that contains a large number of ceramic layers stacked on one another is also referred to as a so-called multimorphous piezoelectric element. By contrast with this, a piezoelectric element in which the stacks are replaced by a single coherent ceramic layer is referred to as a bimorphous or as a trimorphous piezoelectric element, depending on whether an intermediate layer is disposed between the two ceramic layers or not. A multimorphous piezoelectric element offers the advantage, by comparison with a bimorphous or trimorphous piezoelectric element, that the same mechanical energy is provided at a lower applied voltage. The reason for this is that the individual ceramic layers of a multimorphous piezoelectric element have a significantly lower thickness than the ceramic layers of a comparable bimorphous or trimorphous piezoelectric element, so that at the same voltage a higher electrical field is established in accordance with E =U/d, E indicating the electrical field, U the applied voltage and d the thickness of the ceramic layer.
Moreover, Published, Japanese Patent Application JP 60-178677 A2 discloses a piezoelectric multimorphous element, the material of the intermediate layer differs from the material of the ceramic layers.
However, because of the many, comparatively thin individual ceramic layers stacked on one another, a multimorphous piezoelectric element disadvantageously exhibits a high susceptibility to fracture and a comparatively low mechanical stability. Therefore, under load, mutual drift of the individual layers often occurs. Furthermore, a multimorphous piezoelectric element, by comparison with a bimorphous or trimorphous piezoelectric element, exhibits a lower efficiency. In addition, its production entails higher costs.
For the aforementioned reasons, hitherto preference was given to a bimorphous or trimorphous piezoelectric element in technical applications, although a multimorphous piezoelectric element may be operated with a lower voltage. Technical applications for a piezoelectric element are, for example, as a piezoelectric printing head for an ink jet printer, as a sound pick-up or generator for a microphone or a loudspeaker, as a sensor for measuring acceleration or pressure, as a measured value pick-up for gas meters and in particular as a flexural transducer, for example as an actuating element in Braille lines, in readers for blind persons, in textile machines, in pneumatic valves, in recording measuring instruments or in non-contacting surface measuring instruments and so on. In this case, a low operating voltage would permit the use of the piezoelectric element even in locations at risk of explosion, such as in the chemical industry or in mining.